Thermally annealed nanodiamond has been functionalized by C-C coupling of the partially graphitized diamond surface using aryl diazonium salts. Depending on the terminal functional groups, the modified bucky diamond nanoparticles show good solubility (up to 0.63 mg mL(-1)) in different solvents. The agglomerate size of the originally strongly bound detonation diamond (>0.5 mu m) is substantially reduced to similar to 20-50 nm by this chemical procedure and without using mechanical techniques such as strong ultrasound or milling. Arylation with functionalized aryl diazonium salts carrying COOH, SO3H, NO2 or bromoethyl groups opens the way for further covalent grafting of organic structures. Arylation with Ar-COOH or Ar-SO3H leads to the formation of stable colloidal solutions in water and physiological media (i.e. PBS buffer), an important prerequisite for biomedical applications. (C) 2010 Elsevier Inc. All rights reserved.